German patent application Ser. No. DE-A 1642 790 describes an installation for decanting water, which installation associates two different types of tank. A first, central tank for decanting and thickening sludge, and a second annular tank of the sludge bed type. This association is advantageous when very great volumes of water are to be treated. Tanks of the first type decant slowly and, to treat liquid at a high flow rate, they need to be of very large area while tanks of the conical sludge bed type are prohibitively tall when liquid is to be treated at a high flow rate.
However, the installation described in the abovementioned patent application requires a system for scraping the sludge in the sludge bed type tank and a system for pumping the sludge from the sludge bed type tank towards the central decanting and sludge thickening tank.
Preferred embodiments of the present invention avoid the need to use these systems by establishing proper natural drainage of the sludge from the sludge bed type tanks into the central tank.